How well do you know your shoalmate? The answer could be life or death

The mesmerising spectacle of fish shoals cannot be understated, be it anything from gobies (Svensson et al., 2000) to killifish (Lee-Jenkins & Godin, 2010). Our fascination with these aggregations makes them a prime candidate for wildlife documentaries, often when they are under siege from predators, such as a sardine shoal being obliterated by a collection of almost every group of large marine predator you could think of (sealions, tuna, shearwaters, copper sharks, common dolphins, Bryde's whale) in the Indian Ocean (BBC, 2018) or a herring shoal being obliterated by a similarly impressive collection of marine predators (guillemots, gulls and humpback whales) in the North Pacific Ocean (BBC, 2009). These dramatic events are natural history at its most dramatic, and while the shoal is normally annihilated in these examples due to the gathering of nature's equivalent of the avengers (also known as predator's multi-species feeding associations) (Gostischa et al., 2021; Purvin, 2015; Stockin et al., 2022), in situations with less predator complexity and intensity, larger shoaling groups begin to show their anti-predator benefits (Ford & Swearer, 2013; Pacher et al., 2025; Stier et al., 2013). As well as predator avoidance, shoaling can also provide other benefits, such as more efficient movement through environments (Crawford et al., 2025), as highlighted in a recent issue (Perry, 2025). The benefits are such that prey fish will form mixed species shoals (Webster et al., 2025), especially in environments where offsetting predation risk facilitates increased foraging opportunities (Paijmans et al., 2020). However, the ideal situation is being in a shoal of your own species, even better when individuals are a similar size (Hoare et al., 2000). Taking this one step further, in this issue, McCormick et al. (2026) demonstrate that intraspecies relatedness is also an important factor for how well a shoal performs in the face of predators. Juvenile spiny Chromis (Acanthochromis polyacanthus) in an experimental arena were startled with a falling metal weight to simulate a predator attack. Compared to groups, solitary fish were less active, and moved more slowly and were more likely to exhibit a delayed response in response to the stimulus. However, the really exciting result came when comparing the group with familiar kin to the group with non-familiar kin. While not all escape responses were better, the familiar kin group showed the highest escape speeds and travelled the greatest distances after the stimulus, giving them a potential edge. McCormick et al. (2026) hypothesise that this could be because familiar individuals are more likely to share information on threats allowing for quicker responses, or that familiar groups may experience less stress when surrounded by known entities. As shoaling behaviour, like many things, has shown to be impacted by warming temperatures (Mukherjee & Bhat, 2024), understanding the fundamental principles involved in predator avoidance, such as familiarity between prey members in a group, is key. An understanding that helps us appreciate the complexity of what we are losing and perhaps helps give us a chance to save it.